Frequency agile transmitter and receiver architecture for DWDM systems

ABSTRACT

A frequency-agile optical transceiver includes a shared local oscillator (LO), a coherent optical receiver and an optical transmitter. The LO operates to generate a respective LO optical signal having a predetermined LO wavelength. The coherent optical receiver is operatively coupled to the LO, and uses the LO signal to selectively receive traffic of an arbitrary target channel of an inbound broadband optical signal. The optical transmitter is also operatively coupled to the LO, and uses the LO to generate an outbound optical channel signal having a respective outbound channel wavelength corresponding to the LO wavelength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to optical communications systems, and inparticular to a frequency agile transmitter and receiver architecturefor Dense Wavelength Division Multiplexed (DWDM) communications systems.

BACKGROUND OF THE INVENTION

Optical communications networks are becoming increasingly popular fordata transmission due to their high bandwidth capacity. Typically, abit-stream is encoded (e.g., using On-Off-Keying—OOK) to generatesequential symbols that are conveyed through a communications channel bya respective optical channel signal. In most cases, the optical channelsignal is generated by a narrow-band optical source (e.g., a narrow-bandlaser) tuned to a desired channel wavelength. At a receiving end of thecommunications channel, an optical receiver detects and decodes thesymbols of the optical channel signal to recover the originalbit-stream. Typically, the receiver is composed of an optical detectorfollowed by electrical signal processing circuitry. The optical detectorconverts the incoming optical channel signal into a correspondingelectrical channel signal. The electrical signal processing circuitry(e.g., Analog-to-Digital (A/D) converter, digital filter, equalizer,Forward Error Correction circuits, etc.) decode the symbols within theelectrical channel signal to recover the bit-stream.

In Wavelength-Division Multiplexed (WDM) and Dense Wavelength-DivisionMultiplexed (DWDM) optical systems, multiple optical channel signals,each of which has a respective different channel wavelength, aremultiplexed into a broadband optical signal which is launched through anoptical fiber. In order to recover any given bit-stream, thecorresponding optical channel signal must be demultiplexed from thebroadband optical signal and directed to a receiver for detection anddata recovery.

Conventional optical demultiplexers utilize a cascade ofwavelength-selective filters, such as Array Waveguide (AWG) or FiberBragg Grating (FBG) filters. Each filter operates to extract lightwithin a narrow band centered about a predetermined filter wavelength,which is chosen to correspond to a specific channel wavelength.Filter-based demultiplexers suffer a disadvantage that their design istightly related to the channel plan of the communications network.Consequently, the channel plan of the system cannot be changed withoutalso replacing every involved optical demultiplexer in the network.

The publication “Polarization Independent Coherent Optical Receiver”, byB. Glance, Journal of Lightwave Technology, Vol. LT-5, No. 2, February1987, proposes a coherent optical receiver for detecting data trafficencoded within an optical signal. Theoretical considerations relating tothe performance and behavior of coherent optical receivers are presentedin “Performance of Coherent Optical Receivers”, by John R. Barry andEdward A Lee, Proceedings of the IEEE, Vol. 79, No. 8, August 1990 and“Fiber-Optic Communications Systems”2^(nd) ed. Govind P. Agrawal, JohnWiley & Sons, New York, 1997, ISBN 0-471-17540-4, Chapter 6. In general,an optical local oscillator (LO) signal is added to a received opticalsignal, and the combined lightwave is directed towards a photodetector.The current produced by the photodetector includes an IntermediateFrequency (IF) signal that is centered at an IF equal to the differencebetween the LO and optical signal frequencies, usually in the microwave(GHz) range, where well established electrical signal processingtechniques can be employed to detect and decode the data traffic.

In principle, coherent optical receivers of this type offer thepossibility of receiving broadband optical signals without suffering thelimitations of conventional filter-based demultiplexing methods. Forexample, the LO may be tuned to translate any desired optical channelfrequency to a predetermined IF to facilitate carrier detection and datarecovery, in a manner directly analogous to radio frequency homodyne,heterodyne and super-heterodyne receivers. With this arrangement,changes in the channel plan of the network (in terms of the number ofchannels and the specific channel wavelengths used) may be accommodated“on the fly” by changing the LO signal wavelength, rather than thereceiver equipment itself.

Another expected benefit of coherent receivers is based on theirextremely narrow-band data detection performance. In particular,electrical signal filtering of the IF signal typically provides strongattenuation of signal components lying outside of a very narrowfrequency band about the predetermined IF, which should enable thereceiver to discriminate between closely spaced wavelength channels of areceived broadband optical signal.

However, coherent optical receivers suffer a limitation in that theirnarrow-band performance renders them highly sensitive to carrier offsetand phase noise. In fact, optimal data recovery is obtained only whenthe channel frequency (in the IF signal) exactly corresponds with thepredetermined IF. As the channel frequency shifts away from thispredetermined value (i.e., as the carrier offset increases), datarecovery performance degrades rapidly. Phase noise in either the LO orreceived optical signals appears as noise in the IF signal, and degradesreceiver performance. In order to avoid this problem, and thereby enablesatisfactory data recovery, very low noise laser sources (for both thetransmitter and the receiver local oscillator) and microwavephase-locked loops are required. This requirement dramatically increasesthe cost of both transmitters and receivers. As a result, coherentoptical receivers are not commonly utilized in modern opticalcommunications networks.

Accordingly, a cost-effective frequency-agile optical transceiverremains highly desirable.

SUMMARY OF THE INVENTION

An object of the invention is to provide a frequency-agile opticaltransceiver for a broadband optical communications system.

Accordingly, an aspect of the present invention provides afrequency-agile optical transceiver, including a shared local oscillator(LO), a coherent optical receiver and an optical transmitter. The LOoperates to generate a respective LO optical signal having apredetermined LO wavelength. The coherent optical receiver isoperatively coupled to the LO, and uses the LO signal to selectivelyreceive traffic of an arbitrary target channel of an inbound broadbandoptical signal. The optical transmitter is also operatively coupled tothe LO, and uses the LO to generate an outbound optical channel signalhaving a respective outbound channel wavelength corresponding to the LOwavelength.

Thus the present invention provides a frequency-agile opticaltransceiver in which a common LO is used for both reception andtransmission functions. In embodiments in which homodyne carrierdetection is used in the coherent optical receiver, the received channeland the generated outbound channel will have substantially the samewavelength (frequency). In other embodiments, the received channel andthe generated outbound channel will be frequency-shifted relative toeach other.

In a two-way optical transmission system, one node can be nominallydesignated as a “master”, and the other node designated as a “slave”.The LO of the slave node can be controlled by a tuning signal derived atthe master node, such that the frequency difference between the two LO'sapproaches 0 Hz in homodyne detection or a specified frequencydifference in heterodyne detection.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a block diagram schematically illustrating principal elementsof a frequency agile optical transceiver in accordance with a firstembodiment of the present invention;

FIGS. 2 a–b is a block diagram schematically illustrating principalelements of a controllable filter usable in embodiments of the presentinvention;

FIGS. 3 a–e illustrate operation of the optical transceiver of FIG. 1for receiving an arbitrary channel of a broadband optical signal usinghomodyne and heterodyne carrier detection;

FIG. 4 is a block diagram illustrating a network node incorporating aplurality of optical transceivers in accordance with the embodiment ofFIG. 1;

FIGS. 5 a–e illustrate transmission operation of the network node ofFIG. 4, in which the optical transceivers utilize homodyne carrierdetection;

FIGS. 6 a–e illustrate transmission operation of the network node ofFIG. 4, in which the optical transceivers utilize heterodyne carrierdetection;

FIG. 7 is a block diagram schematically illustrating principal elementsof a frequency agile optical receiver in accordance with a secondembodiment of the present invention;

FIG. 8 is a block diagram schematically illustrating principal elementsof a frequency agile optical receiver in accordance with a thirdembodiment of the present invention; and

FIG. 9 is a block diagram schematically illustrating principaloperations of a feedback control loop for tuning the respective localoscillators at opposite ends of a two-way optical communications system.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a frequency agile optical transceiver fortransmitting and receiving data traffic through an arbitrary channel ofa broadband optical signal. FIG. 1 is a block diagram schematicallyillustrating principal elements of a frequency agile optical transceiverin accordance with a first embodiment of the present invention.

As shown in FIG. 1, a frequency agile optical transceiver 2 inaccordance with the present invention generally comprises a coherentoptical receiver 4 cascaded with a controllable IF filter 6 forselectively receiving traffic of a desired “target” wavelength channelof an inbound broadband optical signal 8; an optical transmitter 10 forgenerating an outbound optical channel signal 12 for transmission; ashared local oscillator (LO) 14 for supplying a local oscillator opticalsignal 22 to both the coherent optical receiver 4 and the opticaltransmitter 10; and a controller 18 for controlling performance of boththe controllable IF filter 6 and the LO 14.

The local oscillator (LO) 14 is preferably provided as a tunable narrowband laser, which operates in response to an LO control signal 20produced by the controller 18 to generate a local oscillator opticalsignal 22 having a predetermined LO wavelength. The LO optical signal 22is split into an Rx and a Tx LO signal paths 24 and 26. The Rx LO signalpath 24 is coupled to the coherent optical receiver 4 to facilitatecarrier detection of the target channel within the inbound broadbandoptical signal 8. The Tx LO signal path 26 is coupled to the transmitter10 and modulated to generate the outbound optical channel signal 12.

The coherent optical receiver 4 operates to generate an IntermediateFrequency (IF) signal 28, in which signal components of the targetchannel are centered about a predetermined IF frequency. Thus thecoherent optical receiver 4 includes an optical coupler 30 (e.g., aconventional 3 dB coupler) for combining the Rx LO optical signal 24 andthe inbound broadband optical signal 8. The combined lightwave 32emerging from the coupler 30 is then directed to a photodetector 34(e.g., a conventional PIN photodiode), which generates an electricalIntermediate Frequency (IF) signal 28 containing a frequency shiftedreplica of the received broadband optical signal 8. The controllable IFfilter 6 operates to isolate signal components of the target channelwithin the IF signal 28, to generate a corresponding received signal 36for clock and data recovery (not shown).

The controllable IF filter 6 can be implemented in various ways,depending on the format of the inbound broadband optical signal 8, andthe capabilities of downstream clock and data recovery circuitry (notshown). For example, in embodiments in which the inbound broadbandoptical signal 8 is formatted with uniform channel bandwidths (such as,for example, the International Telecommunications Union (ITU) 50 GHzgrid), the filter 6 may be provided with a fixed filter characteristichaving a predetermined center frequency, and a bandwidth that isselected to encompass the signal components corresponding to a singlewavelength channel within the IF signal 28. In other cases, the filter 6may be provided with a variable filter characteristic, in which thecenter frequency and/or bandwidth may be adjusted, for example inresponse to a filter control signal 38 generated by controller 18.

FIG. 2 a is a block diagram schematically illustrating principalelements of a controllable IF filter 6 usable in the present invention.As shown in FIG. 2 a, the controllable IF filter 6 is provided as ananalog anti-aliasing filter 40, an analog-to-digital converter (ADC) 42,and a digital filter 44. In this case, the sampling rate (fs) of the ADC42, and the bandwidth of the anti-aliasing filter 40 are selected tosatisfy the Nyquist sampling theorem for signal components correspondingto the target wavelength channel within the IF signal 28. In someembodiments, this may be accomplished by selecting the sampling rate(fs) and the bandwidth of the anti-aliasing filter 40 to satisfy theNyquist sampling theorem for the expected widest bandwidth channel to bereceived by the transceiver 2. The bandwidth of the IF filter 6 iscontrolled by selecting and/or programming the filter characteristic ofthe digital filter 44, in a manner well known in the art. The IF filter6 of FIG. 2 a is particularly suitable for embodiments of the inventionin which homodyne detection is used (as described in detail below),because the required low-pass filer characteristics required to isolatetraffic of any desired wavelength channel simplifies the designrequirements of the anti-aliasing filter 40 and the ADC 42.

FIG. 2 b is a block diagram schematically illustrating principalelements of an alternative controllable IF filter 6 usable in thepresent invention. As shown in FIG. 2 b, the controllable IF filter 6 isprovided as a set of two or more parallel analog filter blocks 46coupled between a pair of switch circuits 48. In the illustratedembodiment, four filter blocks 46 are provided, although more, or fewerfilter blocks may be used, as desired. Each filter block 46 is designedas a conventional analog filter network having a respectivepredetermined filter characteristic. The filter characteristic of eachfilter block 46 can be designed to suit the requirements of an expectedchannel IF and bandwidth within the IF signal 28. For example, in thecase of homodyne detection, the filter blocks 46 can all be provided aslow-pass filters, each having a respective different cut-off frequencyfc. In the case of heterodyne detection, the filter blocks 46 can all beprovided as band-pass filters, each having a common center frequency(corresponding to the expected channel IF) and a respective differentpass band width. In both cases, a wavelength channel of virtually anybandwidth can be accommodated by the controllable IF filter 6 byselecting the filter block 46 for which the filter characteristic mostclosely matches the requirements of the wavelength channel, and thencontrolling the switch circuits 48 to route the IF signal 28 to theselected filter block 46. The IF filter 6 of FIG. 2 b is particularlysuitable for embodiments of the invention in which a limited number ofdifferent channel bandwidths are expected in the network.

Referring back to FIG. 1, the optical transmitter 10 operates tomodulate the Tx LO optical signal 26 using an output signal 50 andthereby generate the outbound optical channel signal 12 for transmissionthrough the network. This functionality can be accomplished using one ormore optical modulators (such as Mach-Zehnder modulators) in a mannerwell known in the art.

If desired, a polarization controller 52 can be used to control thepolarization state of the broadband optical signal 8, and thereby ensurealignment between the polarization states of the received broadband andRx LO optical signals 8 and 24 within the optical coupler 30. Inaddition, a controllable phase shifter 54 may be used to ensure phasealignment between the received broadband and Rx LO optical signals 8 and24 within the optical coupler 30. If desired, a group filter 56 may beprovided to filter the inbound broadband optical signal 8, so as toreduce the total optical energy input to the photodetector 34. This canbe useful to reduce optical noise and prevent saturation of thephotodetector 34.

FIG. 3 a shows a typical optical spectrum of the inbound broadbandoptical signal 8. Following a conventional ITU 50 GHz grid, thebroadband optical signal 8 is divided into multiple wavelength channels58 on a 50 GHz spacing. This channel plan facilitates multiplexing anddemultiplexing of individual wavelength channels 58 using conventionalfilter based optical multiplexing and demultiplexing techniques, and istolerant of moderate phase noise in optical transmitter optical sources.As may be seen in FIG. 3 a, the optional group filter 56 (FIG. 1) has aband-pass filter characteristic 60 that defines a channel group 62containing the target wavelength channel 64. With this arrangement, thegroup filter 56 attenuates components of the inbound broadband opticalsignal 8 lying outside the channel group 62. Consequently, the opticalenergy received by the photodetector 34 is reduced to the selectedchannel group 62 and the Rx LO signal 24.

An important advantage of the present invention is that the transceiver2 is capable of detecting and isolating traffic of any arbitrarywavelength channel 58 from the inbound broadband optical signal 8. Theability to receive traffic having an arbitrary center wavelength (atleast within the tuning range of the local oscillator 14) is an inherentfunction of conventional coherent optical receivers. However, thetransceiver 2 of the present invention is further capable of receivingtraffic having any arbitrary channel bandwidth. This functionality isprovided by the controllable IF filter 6, as will be described ingreater detail below. Accordingly, while the standard ITU grid is usedin conventional optical networks (and thus used for illustrativepurposes in FIG. 3), a regular channel spacing is not necessary for thepresent invention. In fact, in networks in which the optical transceiver2 of the present invention is utilized, any arbitrary mix of high andlow bandwidth wavelength channels, and any arbitrary channel spacing,may be used.

FIGS. 3 b and 3 c illustrate operation of the transceiver 2, whenhomodyne carrier detection is used. In this case, the LO 14 is tuned tomatch the channel wavelength of the target channel 64. As a result,signal components of the IF signal 28 corresponding to the targetchannel 64 will be centered about an “intermediate” frequency 66 a ofzero Hz. In conventional radio-communications terminology, the targetchannel 64 has been “downconverted” to baseband. In this case, the IFfilter 6 is provided with a low-pass filter characteristic 68 having acut-off frequency (fc) that is selected to encompass signal componentsof the target channel 64, while other components of the IF signal 28 arestrongly attenuated. This operation yields the cumulative response shownin FIG. 3 c, in which signal components of the target channel 64 havebeen isolated from the IF signal 28, and can be output from thetransceiver 2 as a baseband received signal 36. This received signal 36can then be passed to further conventional signal processing circuitry(not shown), for clock and data recovery in a manner known in the art.

FIGS. 3 d and 3 e illustrate operation of the transceiver 2, whenheterodyne carrier detection is used. In this case, the LO 14 is tunedto maintain a selected difference between the LO signal frequency andthe channel frequency of the target channel 64. As a result, signalcomponents of the IF signal 28 corresponding to the target channel 64will be centered about an intermediate frequency 66 b given by theselected frequency difference. In this case, the IF filter 6 can beprovided with a band-pass filter characteristic 70 having a desired(fixed) pass-band center frequency that corresponds with the IF 66 b,and a bandwidth 72 that is selected to encompass signal components ofthe target channel 64. This operation yields the cumulative responseshown in FIG. 3 e, in which signal components of the target channel 64have been isolated from the IF signal 28, and can be output from thetransceiver 2 as a received signal 36. This received signal 36 can thenbe passed to conventional signal processing circuitry (not shown), forclock and data recovery in a manner known in the art.

As may be appreciated, the intermediate frequency 66 can be set to anydesired value, based, for example, on the capabilities of the IF filter6 and/or other signal processing systems (not shown) located downstreamof the IF filter 6. The transceiver 2 can then operate to translate thecenter wavelength (frequency) of any arbitrary channel 58 of thebroadband optical signal 8, as the target channel 64, to the selectedintermediate frequency 66 by suitably controlling the wavelength(frequency) of the LO optical signal 22. Any arbitrary bandwidth of thetarget channel 64 can be accommodated by suitably controlling the filtercharacteristic of the controllable IF filter 6. For example, in the caseof homodyne detection, the cut-off frequency fc can be adjusted to afrequency equivalent to approximately half the desired target channelbandwidth. In the case of heterodyne detection, the width of the filterpassband can be adjusted to correspond with the desired target channelbandwidth.

It will be seen that the Tx LO optical signal 26 serves as the carrierof the outbound optical channel signal 12, for conveying the outputsignal 50 through the communications network. As will be appreciated, inembodiments in which Homodyne detection is used, the wavelength(frequency) of the outbound optical channel signal 12 will correspondwith that of the target channel 64 received by the coherent opticalreceiver 4 and IF filter 6. On the other hand, in embodiments in whichHeterodyne detection is used, an offset will exist between the targetand outbound optical channel wavelengths (frequencies). This phenomenawill be described in greater detail below with reference to FIGS. 4–6.

FIG. 4 illustrates a node 74 of an optical network utilizing a pluralityof optical transceivers 2 a–n of the present invention. Each transceiver2 receives the inbound broadband optical signal 8 and is tuned toreceive a respective different channel 58. Thus, within each transceiver2, the respective LO 14 is tuned such that a respective target channel64 is “downconverted” to the predetermined IF 66, passed by the IFfilter 6, and emerges from the transceiver 2 as a respective channelreceived signal 36. Thus the respective LO 14 of each transceiver 2 willbe tuned to an LO wavelength (frequency) that is unique, at least acrossthe transceivers 2 a–2 n that are receiving the inbound broadbandoptical signal 8.

In embodiments in which homodyne detection is used, the LO signalwavelength (frequency) will correspond with the channel wavelength(frequency) of the respective target channel 64. Because the LO opticalsignal 22 is also used by the transmitter 10 to generate a respectiveoutbound optical channel signal 12, it follows that the outbound channelwavelength will correspond with that of the respective target channel64, as may be seen in FIGS. 5 a–5 d. As shown in FIG. 5 e, the outboundchannel signals 12 from all of the transceivers 2 can then be combined(in a conventional manner) to generate a composite broadband opticalsignal 76 having the same format as that of the received broadbandoptical signal 8. Thus it will be appreciated that the node 74 can bereadily inserted into existing optical communications networks, withoutrequiring modification or replacement of neighboring network equipment.Furthermore, individual optical transceivers 2 of the present inventioncan be inserted into existing network equipment, without requiringmodification or replacement of either neighboring (e.g. conventional)transceivers within the same node, or neighboring network equipmentwithin the network as a whole. These characteristics provide aconvenient migration path for network providers to upgrade their networkequipment.

In embodiments in which heterodyne detection is used, there will be apredetermined difference between the frequencies of the LO signal 22 andthe target channel 64. Because the LO optical signal 22 is also used bythe transmitter 10, the transmit channel wavelength will necessarily beshifted from that of the received target channel 64 by an offset 78, asmay be seen in FIGS. 6 a–6 d. However, because the offset 78 issubstantially equal for all channels, the respective outbound channelsignals 12 from all of the transceivers can still be combined (in aconventional manner) to generate a composite broadband optical signal 76having the same general format as that of the received broadband opticalsignal 8 (as shown in FIG. 6 e). In this case, however, the compositebroadband optical signal 76 will be frequency-shifted relative to theinbound broadband optical signal 8. The fact that the inbound target andoutbound channel wavelengths are different necessarily implies thatneighboring network equipment (e.g. a downstream node receiving thecomposite broadband optical signal 76) must also be designed toaccommodate the differing channel wavelengths. This problem issimplified by recognizing that the inbound and outbound signals 8 and 76are conveyed through different optical fibers. In addition, thepartitioning of the broadband signals 8 and 76 into channel groups 62provides some tolerance to the presence of a frequency offset 78 betweeninbound and outbound channels. However, even with these simplifications,insertion of the node 74 into existing optical communications networksmay require adjustment or replacement of neighboring network equipment.For this reason, embodiments of the present invention utilizing homodynedetection, as illustrated in FIGS. 3 a–c and 5, are preferred overembodiments utilizing heterodyne detection.

As mentioned previously, in order to successfully detect and isolate thedesired target channel 64 within the inbound broadband optical signal 8,it is necessary to ensure that the LO optical signal 22 and the inboundbroadband optical signal 8 are both phase and polarization alignedwithin the optical coupler 30. In the embodiment of FIG. 1, alignment ofpolarization states is provided by means of a controllable polarizationrotator 52 arranged to control the polarization state of the inboundbroadband optical signal 8. Phase alignment can be ensured by means of acontrollable phase shifter 54. The use of a single optical detector 34means that the receiver 4 of FIG. 1 is suitable for receiving On-OffKeying (OOK), Binary Phase shift Keying (BPSK) or Differential Phaseshift Keying (DPSK) encoded optical signal traffic. However, thereceiver 4 of FIG. 1 will be largely insensitive to polarizationdependent content of the inbound broadband optical signal 8. Thus, forexample, the recieved signal 36 generated by the IF filter 6 of FIG. 1will not permit accurate data recovery of traffic encoded within thetarget channel 64 using polarization multiplexing, polarizationinterleaving or quadrature modulation schemes. FIG. 7 is a block diagramshowing an enhanced frequency agile transceiver 2 a which overcomesthese limitations.

As shown in FIG. 7, the frequency agile transceiver 2 a operates byseparating the inbound broadband optical signal 8 into orthogonalpolarization modes, each of which is sub-divided into a respective pairof components. Each component is then supplied to a respective coherentoptical receiver 4 and IF filter 6 closely similar to that of theembodiment of FIG. 1. Thus the transceiver 2 a includes a polarizationbeam splitter 80 for separating the inbound broadband optical signal 8into orthogonal polarization modes, denoted by H and V in FIG. 7. Thisstep has an additional benefit in that it fixes the polarization stateof the H and V polarization modes, so that a dynamic polarizationcontroller 52 (FIG. 1) is not required. If desired, however, apolarization controller can be used upstream of the polarization beamsplitter 80, in order to align the polarization of the inbound broadbandoptical signal 8 to a principal axis of the polarization beam splitter80. Each of the H and V polarization modes is divided into a pair ofsignal components H1,H2 and V1,V2, each of which is supplied to arespective coherent optical receiver 4.

Similarly, the Rx LO optical signal 24 is divided into orthogonalpolarization modes, denoted by RH and RV in FIG. 7. Each of the RH andRV polarization modes is divided into a pair of signal componentsRH1,RH2 and RV1,RV2, each of which is supplied to the optical coupler 30of a respective coherent optical receiver 4.

Each coherent optical receiver 4 and IF filter 6 combination isconfigured to operate as described above with respect to the embodimentof FIG. 1. The only difference in this case is that one signal componentof each polarization mode of the inbound broadband optical signal 8 (inthis case, components H1 and V1) is combined with correspondingcomponents of the Rx LO optical signal 24 (i.e. RH1 and RV1), asdescribed above with respect to FIG. 1, while the other signal componentof the inbound broadband optical signal 8 (H2 and V2) is combined with a90° phase delayed version of the Rx LO optical signal 24 (i.e. RH2 andRV2). This enables effective carrier detection of the target channel 64,independently of the phase relationship between the inbound broadbandoptical signal 8 and the LO optical signal 22.

As may be seen in FIG. 7, the transceiver 2 a generates a receivedsignal 36 a in the form of a respective pair of received signalcomponents 82 for each polarization mode H, V. Each signal pair 82provides orthogonal (e.g., quadrature) components of the respectivepolarization mode H and V, and therefore provides sufficient informationfor the reconstruction of the respective polarization mode H and V ofthe target channel 64. Taken together, the two received signal pairs 82contain sufficient information for complete reconstruction of the targetchannel 64 of the inbound broadband optical signal 8, includingamplitude, phase, and polarization dependent content. Thus theembodiment of FIG. 7 provides a universal optical transceiver 2 acapable of detecting and isolating traffic of any arbitrary channel 58of an inbound broadband optical signal 8, independently of themodulation or multiplexing scheme used to encode the traffic within thetarget channel 64. As in the embodiment of FIG. 1, the received signal36 a can be forwarded to a signal processor (not shown) for clock anddata recovery and/or other system analysis or management functions, in amanner well known in the art.

As mentioned above, the embodiment of FIG. 7 provides a universaloptical transceiver 2 a capable of detecting and isolating traffic ofany arbitrary channel 58 of an inbound broadband optical signal 8,independently of the polarization and phase of the inbound opticalsignal 8, and independently of the modulation or multiplexing schemeused to encode traffic within the target channel 64. In manyconventional networks, however, optical signals are transmitted withlinear polarization, conventional chromatic multiplexing is used, andtraffic is encoded using quadrature modulation. In this case, asimplified version of the transceiver 2 a of FIG. 7 can be used, asshown in FIG. 8. The simplified transceiver 2 b of FIG. 8 is similar tothe universal transceiver 2 a of FIG. 7, in that the inbound broadbandoptical signal 8 is divided into a pair of components, each of which issupplied to a respective optical receiver 4 and IF filter 6. One of thecomponents is combined with the Rx LO optical signal 24 (as describedabove with respect to FIG. 1), while the other signal component iscombined with a 90° phase delayed version of the Rx LO optical signal 24a. As in the embodiment of FIG. 7, this arrangement enables effectivecarrier detection independently of the phase relationship between theinbound broadband optical signal 8 and the LO optical signal 22.However, because the inbound broadband optical signal 8 was launchedwith a linear polarization, only one pair of optical receivers 4 and IFfilters 6 are required. In addition, the polarization beam splitter 80of FIG. 7 can be eliminated, in favor of a polarization controller 52,which operates to align the polarization of the inbound broadbandoptical signal 8 with the Rx LO optical signals 24, and 24 a.

It should be noted that because the received signal produced by thecoherent optical receiver 4 and IF filter contains sufficientinformation for complete reconstruction of signal components within theIF signal 28, conventional digital signal processing techniques can beused to accomplish effective data recovery, even in the presence ofmoderate phase noise in the LO optical signal 22 and/or the inboundbroadband optical signal 8. In embodiments in which homodyne detectionis used, expensive microwave phase-lock-loops are not required toaccomplish this operation. Additionally, because the receiver 4 and IFfilter 6 of the present invention is capable of down-converting andisolating traffic of any arbitrary channel 58 of the inbound broadbandoptical signal 8, changes in the channel plan of the opticalcommunications network can be accommodated without changing any of thereceiver hardware. In some cases, deployment of the frequency agiletransceiver 2 of the present invention may also allow network nodes tobe provisioned with a smaller number of transceivers, because it is nolonger necessary to provide a separate transceiver for each wavelengthchannel of the network.

As will be appreciated, the received signal 36 generated by the IFfilter 6 will contain subscriber data conveyed through the opticalcommunications system, as well as noise. Various known signal processingtechniques can be used to recover the subscriber data from the receivedsignal 36. Such signal processing may, for example, includeequalization, data detection and forward error correction. As is knownin the art, each of these processing techniques yield information (suchas Bit Error Rate, eye opening, signal power etc.) which may be used toderive a tuning signal for controlling the local oscillator 14. Inaccordance with the present invention, this functionality is extended toenable control of the local oscillators at opposite ends of a two-waycommunications link. This operation is described below with reference toFIG. 9.

As shown in FIG. 9, a two-way optical communications system comprises apair of transceivers 2 at opposite ends of an optical link. One of thetransceivers 2 a is nominally designated as a “master”, while the othertransceiver 2 b is designated as a slave. Both transceivers are providedwith a conventional signal processor 84 which operates to extract thesubscriber data from the received signal 36. Signal quality information(e.g. Bit Error Rate, eye opening, signal power etc.) for bothtransceivers 2 a and 2 b is then detected (at 86) and used (at 88) toderive tuning signals for both transceivers 2.

Thus, for example, at the master transceiver 2 a, signal qualityinformation 90 a obtained by the local signal processor 84 a can bedetected (at 86) and supplied to a processor 88. Corresponding signalquality information 90 b obtained by the signal processor 84 b at the“slave” transceiver 2 b is transmitted to the master transceiver 2 a(e.g. using control channel signaling), detected (at 86) and supplied toa processor 88. Based on the two sets of signal quality information 90 aand 90 b, the processor 88 can then derive respective tuning signals 92for the master and slave transceivers 2 a and 2 b. In particular, the“master” tuning signal 92 a can be derived to set a desired frequency ofthe “master” LO signal 22 a; while the “slave” tuning signal 92 b isderived to define a desired frequency difference between the master andslave LO signals 22 a and 22 b. Deriving both tuning signals 92 a and 92b at a signal processor 88 has an advantage that it enables jointoptimization of the performance of both the master and slavetransceivers 2 a and 2 b. In the case of homodyne detection, the slavetuning signal 92 b would be derived so that the frequency differenceapproaches zero Hz. Alternatively, for heterodyne detection, the slavetuning signal 92 b would be derived so that the frequency differenceapproaches the desired frequency offset 78 between the LO frequency andthe inbound optical signal 8. In either case, the algorithm implementedto derive the master and slave tuning signals 92 must account for thepropagation delays involved in conveying first the slave signal qualityinformation 90 b to the master transceiver 2 a, and then transmittingthe slave tuning signal 92 b back to the slave transceiver 2 b. Variousmethods of accomplishing this (such as by imposing delays on the mastersignal quality information 90 a and the master tuning signal 92 a) willbe readily apparent to those of ordinary skill in the art, and thus willnot be described in greater detail.

The embodiment(s) of the invention described above is (are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A frequency-agile optical transceiver comprising: a local oscillator(LO) for generating a respective LO optical signal having apredetermined LO wavelength; a coherent optical receiver operativelycoupled to the LO, for generating an Intermediate Frequency (IF) signalcontaining traffic of a target channel of an inbound broadband opticalsignal, based on the LO optical signal; a controllable IF filter forselectively isolating signal components of the target channel within theIF signal; and an optical transmitter operatively coupled to the LO forgenerating an outbound optical channel signal having a respectiveoutbound channel wavelength corresponding to the LO wavelength; whereinthe LO is responsive to a tuning signal received from a secondtransceiver to maintain a predetermined frequency difference between theLO optical signal and a respective local oscillator of the secondtransceiver.
 2. A transceiver as claimed in claim 1, wherein the localoscillator comprises a tunable narrow-band laser.
 3. A transceiver asclaimed in claim 1, wherein the coherent optical receiver comprises: anoptical coupler for combining the LO optical signal with the inboundbroadband optical signal to generate a combined lightwave; and aphotodetector for detecting the combined lightwave and generating theIntermediate Frequency (IF) signal.
 4. A transceiver as claimed in claim1, wherein the controllable IF filter comprises: an analog to digitalconverter (ADC) for sampling the IF signal at a predetermined samplerate, and for generating a corresponding digital IF signal; and adigital filter for filtering the digital IF signal in accordance with aselected filter function.
 5. A transceiver as claimed in claim 4,wherein the predetermined sample rate of the ADC is selected to satisfythe Nyquist criterion for signal components of the target channel withinthe IF signal.
 6. A transceiver as claimed in claim 4, wherein thedigital filter comprises any one of: an Infinite Impulse Response (HR)filter; a Finite Impulse Response (FIR) filter; and a Fast FourierTransform (FFT) filter.
 7. A transceiver as claimed in claim 4, furthercomprising a controller for controlling the filter function to adjust atleast a passband width of the digital filter.
 8. A transceiver asclaimed in claim 1, wherein the controllable IF filter comprises: a setof two or more filter blocks, each filter block having a respectivepredetermined filter characteristic; and a switch for selectivelysupplying the IF signal to one of the filter blocks.
 9. A transceiver asclaimed in claim 8, wherein the switch is responsive to a switch controlsignal from a controller, to dynamically supply the IF signal to aselected filter block.
 10. A transceiver as claimed in claim 3, whereinthe coherent optical receiver performs homodyne carrier detection, andthe controllable filter comprises a controllable low-pass filter havinga selected cut-off frequency.
 11. A transceiver as claimed in claim 10,wherein the controllable low-pass filter is responsive to a filtercontrol signal to selectively control the cut-off frequency inaccordance with a bandwidth of the target channel.
 12. A transceiver asclaimed in claim 3, wherein the coherent optical receiver performsheterodyne carrier detection, and the controllable filter comprises acontrollable band-pass filter responsive to a filter control signal toselectively control the pass-bandwidth in accordance with a bandwidth ofthe target channel.
 13. A transceiver as claimed in claim 1, wherein thetransmitter comprises an optical modulator for modulating the LO opticalsignal in accordance an output signal to generate the outbound opticalchannel signal.
 14. A transceiver as claimed in claim 1, furthercomprising: means for detecting at least one signal parameter of thetarget channel; a processor for deriving first and second tuning signalsbased on the detected signal parameters; and means for sending at leastone of first and second tuning signals to the second transceiver.
 15. Atransceiver as claimed in claim 14, wherein the means for detecting atleast one signal parameter comprises a signal processor for processingthe isolated signal components of the target channel, and for generatingat least one parameter indicative of a quality of the target channel.16. A transceiver as claimed in claim 14, wherein the signal parametercomprises any one or more of: signal power; bit error rate; and eyeopening.
 17. A transceiver as claimed in claim 1, wherein thetransceiver is coupled to the second transceiver via a bi-directionaloptical link.